The calcifying phytoplankton species,coccolithophores,have their calcified coccoliths around the cells,however,their physio-logical roles are still unknown.Here,we hypothesized that the coccoliths may play a certain role in reducing solar UV radiation(UVR,280-400 nm) and protect the cells from being harmed.Cells of Emiliania huxleyi with different thicknesses of the cocco-liths were obtained by culturing them at different levels of dissolved inorganic carbon and their photophysiological responses to UVR were investigated.Although increased dissolved inorganic carbon decreased the specific growth rate,the increased coccolith thickness significantly ameliorated the photoinhibition of PSII photochemical efficiency caused by UVR.Increase by 91%in the coccolith thickness led to 35%increase of the PSII yield and 22%decrease of the photoinhibition of the effective quantum yield(ΦPSII) by UVR.The coccolith cover reduced more UVA(320-400 nm) than UVB(280-315 nm) ,leading to less inhibition per energy at the UV-A band.
We carried out short term pCO2/pH perturbation experiments in the coastal waters of the South China Sea to evaluate the combined effects of seawater acidification (low pH/high pCO2) and solar UV radiation (UVR,280-400 nm) on photosynthetic carbon fixation of phytoplankton assemblages. Under photosynthetically active radiation (PAR) alone treatments,reduced pCO2 (190 ppmv) with increased pH resulted in a significant decrease in the photosynthetic carbon fixation rate (about 23%),while enriched pCO2 (700 ppmv) with lowered pH had no significant effect on the photosynthetic performance compared to the ambient level. The apparent photosynthetic efficiency decreased under the reduced pCO2 level,probably due to C-limitation as well as energy being diverged for up-regulation of carbon concentrating mechanisms (CCMs). In the presence of UVR,both UV-A and UV-B caused photosynthetic inhibition,though UV-A appeared to enhance the photosynthetic efficiency under lower PAR levels. UV-B caused less inhibition of photosynthesis under the reduced pCO2 level,probably because of its contribution to the inorganic carbon (Ci)-acquisition processes. Under the seawater acidification conditions (enriched pCO2),both UV-A and UV-B reduced the photosynthetic carbon fixation to higher extents compared to the ambient pCO2 conditions. We conclude that solar UV and seawater acidification could synergistically inhibit photosynthesis.
Marine photosynthesis drives the oceanic biological CO2 pump to absorb CO2 from the atmosphere, which sinks more than one third of the industry-originated CO2 into the ocean. The increasing atmos-pheric CO2 and subsequent rise of pCO2 in seawater, which alters the carbonate system and related chemical reactions and results in lower pH and higher HCO3- concentration, affect photosynthetic CO2 fixation processes of phytoplanktonic and macroalgal species in direct and/or indirect ways. Although many unicellular and multicellular species can operate CO2-concentrating mechanisms (CCMs) to util-ize the large HCO3- pool in seawater, enriched CO2 up to several times the present atmospheric level has been shown to enhance photosynthesis and growth of both phytoplanktonic and macro-species that have less capacity of CCMs. Even for species that operate active CCMs and those whose photo-synthesis is not limited by CO2 in seawater, increased CO2 levels can down-regulate their CCMs and therefore enhance their growth under light-limiting conditions (at higher CO2 levels, less light energy is required to drive CCM). Altered physiological performances under high-CO2 conditions may cause genetic alteration in view of adaptation over long time scale. Marine algae may adapt to a high CO2 oceanic environment so that the evolved communities in future are likely to be genetically different from the contemporary communities. However, most of the previous studies have been carried out under indoor conditions without considering the acidifying effects on seawater by increased CO2 and other interacting environmental factors, and little has been documented so far to explain how physi-ology of marine primary producers performs in a high-CO2 and low-pH ocean.
WU HongYan1, ZOU DingHui1 & GAO KunShan2 1 Marine Biology Institute, Shantou University, Shantou 515063, China
